Fluvial Channel Belt Reservoirs

Abstract

Modern rivers are commonly classified as meandering or braided, but this distinction poorly differentiates the range of interval heterogeneities observed in fluvial channel-belt reservoirs. The problem with this division applied to reservoir type is that class definition is based on unrelated variables (sinuosity in one case, and number of active channel threads in the other), and inferences about a range of other variables that are only weakly related (e.g., mean grain size). Large-scale heterogeneity patterns within channel belts are generally not channel-shaped features, but rather reflect bodies formed as channel segments migrate and then are cut off. These bodies (“storeys”) generally scale to formative river discharge (controlling channel width & depth and the downstream length of adjacent bars). The sinuosity of individual channel segments (before cutoff) defines the width/length ratio of these bodies and internal grain size patterns. Deposits within storeys can be divided into different depositional zones with distinct lateral grain-size trends across the channel bed (which can become vertical trends within the deposits by Walters' law shifts in bed position): inner-bank (bar), concave bank, and abandonment fill. Inward-fining across the inner-bank zone bed becomes more pronounced with distance downstream along a channel bend and channel sinuosity. Upward-fining deposits are preferentially preserved when a channel bend migrates more downstream relative to rates of expansion. Concave bank zone deposits are highly variable depending on whether deposits form due to eddy aggradation or downstream accretion. Channel-fill-zone grain-size trends depend on rates of channel segment abandonment and vertical aggradation vs. lateral-fill deposition. The width of a channel belt formed by a river of given discharge increases with the sinuosity of individual channel segments and the number of storeys laterally stacked during the sum of channel-bend expansion and cutoff events before river avulsion. Connectivity patterns of subsurface fluid flow along a channel belt depends on storey internal character, lateral stacking pattern, net aggradation, and the width spanned by the final fill formed during belt avulsion.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015